refactor: moving ui.extends to core

* Settings and Singletons moved

core/singleton.py

@@ -0,0 +1,18 @@
+from typing import Generic, TypeVar
+
+from PySide6.QtCore import QObject
+
+T = TypeVar("T")
+
+
+class Singleton(type, Generic[T]):
+    _instance = None
+
+    def __call__(cls, *args, **kwargs) -> T:
+        if cls._instance is None:
+            cls._instance = super().__call__(*args, **kwargs)
+        return cls._instance
+
+
+class QSingleton(type(QObject), Singleton):
+    pass